West Virginia Gov. Joe Manchin III (D) has the authority to appoint Byrd's interim replacement and decide whether to call for a special election.

If a vacancy is declared more than 30 months before the expiration of a senator's term the appointee serves until a special election is held, according to the state's election code.

Byrd passed away early Monday. His ninth term was to end Jan. 3, 2013.

If Manchin declares Byrd's seat vacant before July 3, according to the state code, there would have to be a special election, likely set for November.

Declaring the seat vacant after July 3 would mean the interim senator would serve until January 2013.